APOPLASM
C. Iron
V. ALUMINIUM TOXICITY
leakiness of membranes (19). Zinc has essential functions in the stabilization of membranes (236,237) and in preventing oxidative membrane damage as a metal component of superoxide dismutase, which is part of the free-radical scavenging system of higher plants (238). It is, however, as yet unknown whether this kind of Zn deficiency-induced root exudation has any impact on mobilization of Zn or other micronutrients in the rhizosphere.
Comparatively high mobility of Cd in soils associated with high rates of uptake and accumulation in some plant species is an important aspect from the ecotoxicological point of view. Cd mobilization in soils can be mediated by rhizo- sphere acidification (223) but to some extent also by complexation with carboxyl- ates (99,221) or phytosiderophores (209). A comparison of high and low Cd- accumulating genotypes of durum wheat revealed higher levels of carboxylates in the rhizosphere soil of the Cd accumulator (222). Based on these findings, it was concluded that plant availability of Cd may be increased by complexation with root-derived carboxylates. In contrast, Gerke (99) suggested that carboxylate complexation of Cd might decrease plant availability, since only free Cd?+ seems to be taken up by plant roots (229,239). Wallace (225) demonstrated that, in soil- plant systems, solubility and transport to the root uptake sites are likely to be the limiting steps in uptake of cationic microelements by plants.
,mg
n brane ApoDlasm Rhizosphere
Figure 10 Model for mechanisms involved in aluminium (AI) exclusion and detoxifica-
tion at the root apex. (A) Enhanced solubilization of mononuclear AI species from AI oxides and AI silicates in the soil matrix at pH < 5.0. (B) Al-induced stimulation of carboxylate exudation via anion channels, charge-balanced by concomitant release of K+.
(C) Formation of Al-carboxylate complexes in the apoplasm; restricted root uptake and lower toxicity of complexed AI. (D) AI complexation in the mucilage layer (polygalacturo- nates) and with Al-binding polypeptides. Increased accumulation of Al-chelating carbox- ylates in the mucilage layer due to limited diffusion.
(85,86,249); it rapidly drops to baseline levels when AI is removed from treatment solutions (83,85). Al-induced carboxylate exudation can reach a level that is com- parable to the extraordinary high exudation of citrate in P-deficient white lupin (74), even in plant species without increased carboxylate exudation under P-defi- cient conditions, such as Cassia tom (249), wheat (82,242), or potato (Neumann, unpublished). Carboxylate release in Al-stressed plants can be blocked by anion channel antagonists (85,86), suggesting the involvement of an anion channel in the plasma membrane. The release of carboxylate anions may be balanced by an equimolar release of K+ (85). Patch-clamp studies with protoplasts isolated from root tips of wheat revealed the Al-induced activation of a Cl- channel with char- acteristics similar to those of Al-mediated release of malate, but with a slower response to the A1 treatment in some experiments. It remains to be established whether this anion channel also shows permeability toward malate (250). In elec-
trophysiological studies, an AI-induced membrane depolarization, which was not caused by malate efflux, was observed in roots of AI-tolerant but not in Al-sensi- tive genotypes of wheat (251). The authors suggested that this depolarization, together with other as yet unknown factors, might be involved in gating of a
voltage-dependent malate channel in root tips of AI-tolerant wheat lines. Al- though the release of carboxylates in response to AI treatments occurs almost instantaneously, inhibitory effects of cycloheximide suggest that intact protein synthesis is required for carboxylate exudation (85). During short-term exposure to A1 stress (2-3 h ) the internal concentrations of carboxylates in root tips of wheat (242) and buckwheat (86) remained at a constant level, and there were no differences in AI-tolerant and AI-sensitive genotypes (242). In root tips of wheat, the activities of enzymes involved in biosynthesis of malate (PEP carboxylase;
NAD malate dehydrogenase) were also not changed (85). It was concluded that the capacity for carboxylate accumulation in the root tissue is not a limiting factor for Al-induced exudation of carboxylates, at least in short-term experiments, and genotypical variations cannot be explained on basis of differences in the capacity for biosynthesis of carboxylic acids (85,242). However, intense exudation of car- boxylates over longer periods as a prerequisite for an efficient AI detoxification (248) would probably require also increased rates of biosynthesis. Accordingly, AI treatments over 16 h enhanced both the exudation and the internal concentra- tion of citrate in root tips of potato and sunflower (Neumann, unpublished). This is in line with findings of De la Fuente et al. (252), who reported induction of AI tolerance in transgenic tobacco and papaya by constitutive cytosolic expression of a bacterial citrate synthase gene from Pseudot~zorzas aeruginosu, which resulted in higher rates of citrate accumulation in the root tissue and higher rates of citrate exudation. Genetic analysis of the AI-tolerance trait and AI-inducible root exuda- tion of malate in hexaploid wheat revealed a predominant control by one single gene, which may be involved in the signaling of the exudation process (247).
Also, other constituents of root exudates have been implicated in binding of AI and thus in AI detoxification in the rhizosphere (Fig. IO). Examples are the release of AI-binding polypeptides (253,254) and of phosphate anions in wheat (246). A high binding capacity for A1 has also been demonstrated for mucilage, which is preferably released in apical root zones (26,255). AI complexation in the mucilage may be attributed to the presence of polycarboxylates (polygalacturonic acids) as integral constituents ( I ) but also to accumulation of LMW carboxylates and phosphates in the highly viscid mucilage layer (Fig. 10) (256).
Unlike carboxylic acids, the release and also the production of phytosidero- phores in roots of both AI-sensitive and AI-tolerant wheat cultivars was rapidly inhibited in response to AI treatments and seems to be responsible for AI-induced iron chlorosis in wheat (257).
Increased root exudation of amino acids in response to Cd toxicity has been reported for lettuce and white lupin grown in a hydroponic culture system under
axenic conditions (60). Under similar culture conditions, a transient release of organic acids (citric, maleic) was observed after addition of high Cu concentra- tions [50 PM] to the culture medium of sunflower (258), suggesting that stimula- tion of root exudation may be triggered by toxic levels of various metal species, probably as a consequence of impaired integrity of the plasma membrane.
VI. TEMPERATURE
Diffusion-mediated release of root exudates is likely to be affected by root zone temperature due to temperature-dependent changes in the speed of diffusion pro- cesses and modifications of membrane permeability (259,260). This might ex- plain the stimulation of root exudation in tomato and clover at high temperatures.
reported by Rovira (261), and also the increase in exudation of sugars and amino acids in maize, cucumber, and strawberry exposed to low-temperature treatments (S-IO'C), which was mainly attributed to a disturbance in membrane permeabil- ity (259,262). A decrease of exudation rates at low temperatures may be predicted for exudation processes that depend on metabolic energy. This assumption is supported by the continuous decrease of phytosiderophore release in Fe-deficient barley by decreasing the temperature from 30 to 5°C (67).